Liquids, gases, water and wastewaters are commonly transported via pressurized pipelines a majority of which are buried. Once in place, such pipelines are subjected to various forms of damage including external damage, soil movements/instability and third party damage. Additionally, buried pipelines are subject to environmental damage. That is, buried pipelines, especially cast iron and steel pipelines, are susceptible to corrosion.
Buried pipelines usually receive one or more forms of external corrosion protection. A protective coating represents a primary form of protection while cathodic protection (CP) represents a secondary form of protection in some instances. The CP system is designed to protect the external pipeline surface against corrosion at coating defects that inevitably occur as the coating condition tends to deteriorate with time. In such corrosion protection systems, regular inspections are made to assess the rate of change in physical condition of the buried pipeline. Such inspection may provide an estimate of how much longer a pipeline can be expected to operate safely and productively and can also be used to plan for remedial action if this predicted life is below requirement
Various corrosion monitoring/inspection techniques are employed in the pipeline industry, such as test station potential readings and Close Interval Potential Surveys (CIPS). These techniques are aimed at assessing the CP effectiveness of the pipeline between permanent test stations. In this regard, pipelines are equipped with permanent test stations where electronic leads are attached to the pipeline to allow above-ground measuring of pipe-to-soil potential. This potential should be sufficiently cathodic to ensure adequate corrosion protection but not excessively cathodic to produce coating damage and/or hydrogen embrittlement.
In such techniques, an operator establishes an electrical connection to the pipeline by means of an above ground wire that extends between test stations. The pipeline potential is measured with a set of reference electrodes at ground level, positioned directly over the pipeline, at intervals of, for example, about 1 meter. The potentials measured above ground can provide an indication of a breakdown in the protection coating of the buried pipeline. For instance, a change in potential at a given location between testing periods or changes in potential relative to adjacent potentials, may indicate that the protective coating is breaking down or has been breached in the measured region of the pipeline. Common to such pipeline monitoring techniques and corrosion control techniques is the requirement that the pipeline itself to carry a current/voltage.